Electrolyte for self-colored anodization of aluminum and alloys

ABSTRACT

A SELF-COLORED ANODIZATION OF ALUMINUM AND ALLOYS OF ALUMINUM BY USE OF AN AQUEOUS ELECTROLYTE CONTAINING SULPHOMALEIC ACID DISSOLVED THEREIN ALONE OR IN COMBINATION WITH SULPHURIC ACID.

3,699,019 ELECTROLYTE FOR SELF-COLORED ANODIZA- TION OF ALUMINUM AND ALLOYS Richaud Henri, Le Mans, and Michel Ronssos, Lyon, France, assignors to Compagnie Pechiney, Paris, France No Drawing. Original application Feb. 10, 1969, Ser. No. 798,122, now Patent No. 3,565,772. Divided and this application Oct. 19, 1970, Ser. No. 82,223

Int. Cl. C23b 9/02 US. Cl. 204-58 3 Claims ABSTRACT OF THE DISCLOSURE The self-colored anodization of aluminum and alloys of aluminum by use of an aqueous electrolyte containing sulphomaleic acid dissolved therein alone or in combination with sulphuric acid.

This application is a division of our copending application Ser. No. 798,122, filed Feb. 10, 1969, now US. Pat. 3,565,772, entitled Process for Self-Colored Anodization of Aluminum and Alloys.

This invention relates to an electrolyte composition and process for self-coloring aluminum and alloys of aluminum by anodization and to articles produced thereby.

It is known that a coating of naturally colored aluminum or articles made of aluminum or alloys of aluminum can be produced without coloring agents by a process called self-colored anodization. The process comprises anodizing the articles in an electrolyte formed of an aqueous solution of a mixture of organic acids of the family of sulphonated aromatic acids, sulphonated or unsulphonated aliphatic acids and possibly sulphuric acid.

Applicants have successfully used electrolyte compositions such as an aqueous solution of chromotropic acid and sulphomaleic acid or an aqueous solution of Z-naphtol- 3,6-disulphonic acid, sulphomaleic acid and sulphuric acid.

Electrolytes containing sulphonated aromatic acids are relatively expensive. It has been found by applcants that good results can be obtained without such sulphonated aromatic acids when use is made of a properly selected sulphonated aliphatic acid.

It is an object of this invention to provide an electrolyte and process for self-colored anodization of aluminum and alloys of aluminum in which use is made of a sulphonated aliphatic acid in the electrolyte without the use of expensive sulphonated aromatic acids.

The present invention concerns an electrolyte containing a sulphonated aliphatic acid which permits the aluminum or alloy of aluminum to be produced with a layer of self-colored alumina with shades ranging from pale bronze to black and passing through deeper and deeper bronze and brown shades therebetween. The invention also includes articles treated in accordance with the process.

In accordance with the practice of this invention, the elecitrolyte comprises an aqueous solution of sulphomaleic aci I aH (HOOCCH=C-COOH) preferably in combination with sulphuric acid.

In the preferred practice of this invention, the electrolyte is composed of a solution of from 10 to 300 g./l. and preferably 50 to 200 g./l. of sulphomaleic acid and from to 15 g./l. and preferably 0.5 to 8 g./l. of sulphuric acid in de-ionized water.

Before anodization, the articles can be subjected to various surface treatments in order to prepare the surface with a matt or shiny finish. Matt surfaces can be ob- United States Patent 0 Patented Oct. 17, 1972 tained by chemical or electro-chemical pickling, such, for example, as in a solution containing 50 g./l. of sodium hydroxide at 50 C. for 10 minutes, followed by rinsing in running water, immersion in 36 B nitric acid and then further rinsing with water. A shiny surface can be produced by mechanical polishing or by chemical or electrolytic polishing.

In accordance with the practice of this invention, selfcolored anodization of the aluminum or alloy of aluminum in the electrolyte can be effected either with alternating current or with an undulating current resulting from the superposition of a direct current and alternating current, but preferably with direct current, the voltage at the terminals of the anodization tank being regulated such that the current density is from 0.5 to 10 amperes per sq. drn. of anodized surface and preferably within the range of 1 to 4 a./sq. dm.

During the anodization, the electrolyte is maintained at a temperature within the range of l0 C. to +50 C. and preferably within the range of 15 C. to 30 C. After the current has been passed for approximately /2 to 1 hour, there is obtained generally a 10 to 50 micron thick layer of uniformly colored alumina with more or less dark bronze or even brown tints. After rinsing with water and annealing, the layer is characterized by being very hard and having remarkable light and weather resistance. Sealing of the layer of alumina is effected by immersion for about 30 minutes in boiling water preferably with a salt, such as nickel acetate, dissolved therein in an amount within the range of 0.5 to 2 g./l.

Depending upon the composition of the electrolyte and the alloy, it is possible within a certain range to select the tint of the oxide layer by fixing the two main regulating parameters, which are current density and temperature of the electrolyte. The tint becomes lighter as the current density decreases and as the temperature increases.

When articles having an initial shiny surface are treated, different techniques may be employed to retain this surface during anodization. For example, the shiny surface can be retained when anodization is initiated at low current density, such as 1 a./sq. dm. for about 10 min utes, after which the density may be increased to terminate the anodization reaction at a current density greater than 1.5 a./ sq. dm.

The composition of the aluminum or alloy of aluminum has noticeable influence on the tint of the resulting layer of alumina. For example, when dark tints are desired, an alloy containing manganese should be used. However, it is noteworthy that with the electrolyte of this invention, variation of the aforementioned parameters of temperature and current density makes it possible to promote the appearance of dark tints, such as brown or even black, when use is made of aluminum alloys which do not contain manganese. It is interesting to note that the same results are not secured with other organic electrolytes of similar composition, such as with sulphosuccinic acid.

The following examples are given by way of illustration, but not by way of limitation, of the practice of this invention:

EXAMPLE 1 A shaped part made of A-GS alloy containing, by weight, 0.5% of silicon, 0.5% of magnesium, the remainder being 99.5% per aluminum, is anodized in an aqueous solution containing g./l. of sulphomaleic acid and 5 g./ l. of sulphuric acid. Anodization is effected .at a temperature of 20 C. with a direct current density of 1.5 a./.sq. dm. for 40 minutes. After sealing, the resulting layer of alumina, which is 18 microns thick, is of a light bronze, uniform color which is resistant to ultraviolet rays.

3 EXAMPLE 2 A shaped part made of A-GS alloy having the same composition as the alloy of Example 1 is anodized in an aqueous solution containing 150 g./1. of sulphomaleic acid and 3 g./l. of sulphuric acid and maintained at 20 C. with a direct current density of 2.5 -a./sq. dm. for 30 minutes. After sealing, the layer of alumina, which is 20 microns thick, is of a dark bronze, uniform color which is resistant to ultra-violet rays.

EXAMPLE 3 A sheet made of A-GO 6 alloy containing, by weight, 0.6% of magnesium, the remainder being 99.7% pure aluminum, is anodized in an aqueous solution containing 80 g./l. of sulphomaleic acid and 6 g./l. of sulphuric acid and maintained at 21 C., with a current density of 1.5 a./sq. dm. for 40 minutes. After sealing, the layer of alumina, which is 18 microns thick, is of a light bronze, uniform color which is resistant to ultra-violet rays.

EXAMPLE 4 A shaped part made of A-SGM alloy containing, by weight, 1% of silicon, 1% of magnesium, 1% of manganese, the remainder being 99.5% pure aluminum, is anodized in an aqueous solution containing 120 g./l. of sulphomaleic acid and 4 g./l. of sulphuric acid, at a temperature of 20 C., with a current density of 2.5 a./sq. dm. for 60 minutes. The resulting layer of alumina, which is microns thick, is of a black, uniform color which is resistant to ultra-violet rays.

It will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. An electrolyte composition for self-colored anodization of aluminum and alloys of aluminum comprising an aqueous solution containing sulphomaleic acid in an 'amount within the range of 10 to 300 g./l. and sulphuric acid in an amount within the range of .5 to 15 g./l.

2. An electrolyte composition as claimed in claim 1 in which the sulphomaleic acid is present within the range of to 200 g./l.

3. An electrolyte composition as claimed in claim 1 in which the sulphuric acid is present in an amount within the range of 0.5 to 8 g./l.

References Cited UNITED STATES PATENTS 2,028,091 1/1936 Jaeger 260-481 2,413,814 1/1947 Groote et al 260-4 81 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

